Gate pullback at ends of high-voltage vertical transistor structure

ABSTRACT

In one embodiment, a transistor includes a pillar of semiconductor material arranged in a racetrack-shaped layout having a substantially linear section that extends in a first lateral direction and rounded sections at each end of the substantially linear section. First and second dielectric regions are disposed on opposite sides of the pillar. First and second field plates are respectively disposed in the first and second dielectric regions. First and second gate members respectively disposed in the first and second dielectric regions are separated from the pillar by a gate oxide having a first thickness in the substantially linear section. The gate oxide being substantially thicker at the rounded sections. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure.

This application is a continuation of application Ser. No. 12/583,745,filed Aug. 25, 2009, now U.S. Pat. No. 8,222,691, issued Jul. 17, 2012,which is a continuation of application Ser. No. 11/707,820, filed Feb.16, 2007, now U.S. Pat. No. 7,595,523, issued Sep. 29, 2009, entitled,“Gate Pullback at Ends of High-Voltage Vertical Transistor Structure”,both of which are assigned to the assignee of the present application.

TECHNICAL FIELD

The present disclosure relates to semiconductor device structures andprocesses for fabricating high-voltage transistors.

BACKGROUND

High-voltage, field-effect transistors (HVFETs) are well known in thesemiconductor arts. Many HVFETs employ a device structure that includesan extended drain region that supports or blocks the appliedhigh-voltage (e.g., several hundred volts) when the device is in the“off” state. In a conventional vertical HVFET structure, a mesa orpillar of semiconductor material forms the extended drain or driftregion for current flow in the on-state. A trench gate structure isformed near the top of the substrate, adjacent the sidewall regions ofthe mesa where a body region is disposed above the extended drainregion. Application of an appropriate voltage potential to the gatecauses a conductive channel to be formed along the vertical sidewallportion of the body region such that current may flow vertically throughthe semiconductor material, i.e., from a top surface of the substratewhere the source region is disposed, down to the bottom of the substratewhere the drain region is located.

In a traditional layout, a vertical HVFET consists of long continuoussilicon pillar structure that extends across the semiconductor die, withthe pillar structure being repeated in a direction perpendicular to thepillar length. One problem that arises with this layout, however, isthat it tends to produce large warping of the silicon wafer during hightemperature processing steps. In many processes, the warping ispermanent and large enough to prevent the wafer from tool handlingduring subsequent processing steps. Additionally, gate oxide weakness inthe rounded end portions of the transistor layout can result in gateoxide breakdown voltage and reliability problems.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will be understood more fully from the detaileddescription that follows and from the accompanying drawings, whichhowever, should not be taken to limit the invention to the specificembodiments shown, but are for explanation and understanding only.

FIG. 1 illustrates an example cross-sectional side view of a verticalHVFET structure.

FIG. 2A illustrates an example layout of the vertical HVFET structureshown in FIG. 1.

FIG. 2B is an expanded view of one portion of the example layout shownin FIG. 2A.

FIG. 3A illustrates another example layout of the vertical HVFETstructure shown in FIG. 1.

FIG. 3B is an expanded view of one portion of the example layout shownin FIG. 3A.

FIG. 4A illustrates yet another example layout of the vertical HVFETstructure shown in FIG. 1.

FIG. 4B is an expanded view of one portion of the example layout shownin FIG. 4A.

FIG. 5 illustrates an example layout of a wafer with die-to-diecheckerboarding of HVFETs.

FIG. 6 illustrates an example layout of a wafer with die-to-diecheckerboarding of segmented HVFETs.

FIG. 7 illustrates an example layout of a rectangular die withcheckerboarded blocks of HVFET segments.

FIG. 8 illustrates an example gate metal routing layout for the dieshown in FIG. 7.

FIG. 9 illustrates an example gate and source metal routing layout forthe die shown in FIG. 7.

FIG. 10 illustrates an expanded portion of the example layout shown inFIG. 9.

FIG. 11 illustrates an example layout of the rounded end portion of asingle HVFET segment having a structure as shown in FIG. 1.

FIG. 12 illustrates another example layout of the rounded end portion ofa single HVFET segment having a structure as shown in FIG. 1.

DETAILED DESCRIPTION

In the following description specific details are set forth, such asmaterial types, dimensions, structural features, processing steps, etc.,in order to provide a thorough understanding of the present invention.However, persons having ordinary skill in the relevant arts willappreciate that these specific details may not be needed to practice thepresent invention. It should also be understood that the elements in thefigures are representational, and are not drawn to scale in the interestof clarity.

FIG. 1 illustrates an example cross-sectional side view of a verticalHVFET 10 having a structure that includes an extended drain region 12 ofN-type silicon formed on an N+ doped silicon substrate 11. Substrate 11is heavily doped to minimize its resistance to current flowing throughto the drain electrode, which is located on the bottom of the substratein the completed device. In one embodiment, extended drain region 12 ispart of an epitaxial layer that extends from substrate 11 to a topsurface of the silicon wafer. A P-type body region 13 and N+ dopedsource regions 14 a & 14 b laterally separated by a P-type region 16,are formed near a top surface of the epitaxial layer. As can be seen,P-type body region 13 is disposed above and vertically separatesextended drain region 12 from N+ source regions 14 a & 14 b and P-typeregion 16.

In one embodiment, the doping concentration of the portion of epitaxiallayer which comprises extended drain region 12 is linearly graded toproduce an extended drain region that exhibits a substantially uniformelectric-field distribution. Linear grading may stop at some point belowthe top surface of the epitaxial layer 12.

Extended drain region 12, body region 13, source regions 14 a & 14 b andP-type region 16 collectively comprise a mesa or pillar 17 (both termsare used synonymously in the present application) of silicon material inthe example vertical transistor of FIG. 1. Vertical trenches formed onopposite sides of pillar 17 are filled with a layer of dielectricmaterial (e.g., oxide) that makes up dielectric region 15. The heightand width of pillar 17, as well as the spacing between adjacent verticaltrenches may be determined by the breakdown voltage requirements of thedevice. In various embodiments, mesa 17 has a vertical height(thickness) in a range of about 30 μm to 120 μm thick. For example, aHVFET formed on a die approximately 1 mm×1 mm in size may have a pillar17 with a vertical thickness of about 60 μm. By way of further example,a transistor structure formed on a die of about 2 mm-4 mm on each sidemay have a pillar structure of approximately 30 μm thick. In certainembodiments, the lateral width of pillar 17 is as narrow as can bereliably manufactured (e.g., about 0.4 μm to 0.8 μm wide) in order toachieve a very high breakdown voltage (e.g., 600-800V).

In another embodiment, instead of arranging P-type region 16 between N+source regions 14 a & 14 b across the lateral width of pillar 17 (asshown in FIG. 1), N+ source regions and P-type regions may bealternately formed at the top of pillar 17 across the lateral length ofpillar 17. In other words, a given cross-sectional view such as thatshown in FIG. 1 would have either an N+ source region 14, or a P-typeregion 16, that extends across the full lateral width of pillar 17,depending upon where the cross-section is taken. In such an embodiment,each N+ source region 14 is adjoined on both sides (along the laterallength of the pillar) by P-type regions 16. Similarly, each P-typeregion 16 is adjoined on both sides (along the lateral length of thepillar) by N+ source regions 14.

Dielectric regions 15 a & 15 b may comprise silicon dioxide, siliconnitride, or other suitable dielectric materials. Dielectric regions 15may be formed using a variety of well-known methods, including thermalgrowth and chemical vapor deposition. Disposed within each of thedielectric layers 15, and fully insulated from substrate 11 and pillar17, is a field plate 19. The conductive material used to from fieldplates 19 may comprise a heavily doped polysilicon, a metal (or metalalloys), a silicide, or other suitable materials. In the completeddevice structure, field plates 19 a & 19 b normally function ascapacitive plates that may be used to deplete the extended drain regionof charge when the HVFET is in the off state (i.e., when the drain israised to a high voltage potential). In one embodiment, the lateralthickness of oxide region 15 that separates each field plate 19 from thesidewall of pillar 17 is approximately 4 82 m.

The trench gate structure of vertical HVFET transistor 80 comprises gatemembers 18 a & 18 b, each respectively disposed in oxide regions 15 a &15 b on opposite sides of pillar 17 between field plates 19 a & 19 b andbody region 13. A high-quality, thin (e.g., ˜500 Å) gate oxide layerseparates gate members 18 from the sidewalls of pillar 17 adjacent bodyregion 13. Gate members 18 may comprise polysilicon, or some othersuitable material. In one embodiment, each gate member 18 has a lateralwidth of approximately 1.5 μm and a depth of about 3.5 μm.

Practitioners in the art will appreciate that N+ source regions 14 andP-type body region 13 near the top of pillar 17 may each be formed usingordinary deposition, diffusion, and/or implantation processingtechniques. After formation of the N+ source region 38, HVFET 10 may becompleted by forming source, drain, gate, and field plate electrodesthat electrically connect to the respective regions/materials of thedevice using conventional fabrication methods (not shown in the figuresfor clarity reasons).

FIG. 2A illustrates an example layout of the vertical HVFET structureshown in FIG. 1. The top view of FIG. 2A shows a single, discrete HVFETcomprising an upper transistor section 30 a and a lower transistorsection 30 b on a semiconductor die 21. The two sections are separatedby a dummy silicon pillar 32. Each section 30 comprises a plurality of“racetrack” shaped transistor structures or segments, each transistorsegment comprises an elongated ring or oval that includes a siliconpillar 17 surrounded on opposite sides by dielectric regions 15 a & 15b. Pillar 17, itself, extends laterally in the x and y directions toform a continuous, elongated, racetrack-shaped ring or oval. Disposedwithin dielectric regions 15 a & 15 b are respective gate members 18 a &18 b and field plates 19 a & 19 b. Field plate 19 a comprises a singleelongated member that terminates on either end in a rounded fingertiparea. Field plate 19 b, on the other hand, comprises an enlarged ring oroval that encircles pillar 17. Field plates 19 b of adjacent racetrackstructures are shown merged such that they share a common member on aside. By way of reference, the cross-sectional view of FIG. 1 may betaken through cut lines A-A′ of the example layout of FIG. 2A.

It should be understood that in the example of FIG. 2A, each of theracetrack transistor segments has a width (i.e., pitch) in they-direction of approximately 13 μm, a length in the x-direction in arange of about 400 μm to 1000 μm, with a pillar height of about 60 μm.In other words, the length to width ratio of the individual racetracktransistor segments comprising sections 30 a & 30 b is in a range ofabout 30 up to 80. In one embodiment, the length of each racetrackshaped segment is at least 20 times greater than its pitch or width.

Practitioners in the art will appreciate that in the completed devicestructure, patterned metal layers are used to interconnect each of thesilicon pillars 17 of the individual transistor segments. That is, in apractical embodiment, all of the source regions, gate members, and fieldplates are respectively wired together to corresponding electrodes onthe die. In the embodiment shown, the transistor segments in eachsection 30 are arranged in a side-by-side relationship in they-direction substantially across a width of die 21. Similarly, in thex-direction the additive length of the transistor segments of sections30 a & 30 b extend substantially over the length of die 21. In theexample layout of FIG. 2A the width of dielectric regions 15 separatingthe silicon pillars, as well as the width of the field plates, issubstantially uniform across semiconductor die 21. Laying out thetransistor segments with uniform widths and separation distancesprevents the formation of voids or holes following the processing stepsused to conformably deposit the layers that comprise dielectric regions15 and field plates 19.

FIG. 2B is an expanded view of one portion of the example layout shownin FIG. 2A. For purposes of clarity, only pillars 17 and dielectricregions 15 b of each of the transistor segments is represented. Dummysilicon pillar 32 is shown separating the rounded end areas ofdielectric regions 15 b of respective transistor segment sections 30 a &30 b. In other words, the deep vertical trenches that are etched in thesemiconductor substrate to define pillars 17 also define dummy siliconpillar 32. In one embodiment, dummy silicon pillar 32 is made to have awidth in the x-direction (i.e., that separates the transistor segmentsections) that is as small as can be reliably manufactured.

The purpose of segmenting the single die HVFET into sections separatedby dummy silicon pillar 32 is to introduce lengthwise (x-direction)stress-relief in the elongated racetrack shaped transistor segments.Segmenting or breaking the transistor device structures into two or moresections relieves mechanical stress across the length of the die. Thisstress is induced by the oxide regions flanking the pillars and normallyconcentrates at the rounded ends of each racetrack segment. Relievingmechanical stress by segmenting the transistor device structures intotwo or more sections thus prevents undesirable warping of the siliconpillars and damage (e.g., dislocations) to the silicon caused by stress.

It is appreciated that a tradeoff exists between the stress reliefprovided by a highly segmented layout and loss of conduction area. Moresegmentation results in greater stress relief, but at the expense ofconduction area. In general, the greater the vertical height of thepillars and the larger the semiconductor die, the greater the number oftransistor sections or segments that will be required. In oneembodiment, for a 2 mm×2 mm die with 60 μm high pillars, adequate stressrelief is provided in a HVFET with an on-resistance of about 1 ohmutilizing a layout comprising four racetrack transistor sectionsseparated by dummy silicon pillars, each having a pitch (x-direction) ofabout 13 μm and a length (y-direction) of about 450 μm.

In another embodiment, instead of a dummy pillar of silicon to separatepairs of racetrack transistor segments, each pair being located in adifferent section, a dummy pillar comprising a different material may beutilized. The material used for the dummy pillar should have a thermalcoefficient of expansion close to that of silicon, or sufficientlydifferent from that of the dielectric region so as to relieve thelengthwise stress induced by the dielectric regions flanking the siliconpillars.

FIG. 3A illustrates another example layout of the vertical HVFETstructure shown in FIG. 1. FIG. 3B is an expanded view of one portion ofthe example layout shown in FIG. 3A, just showing pillars 17, oxideregion 15 b, and an optional dummy silicon pillar 33. Similar to theembodiment of FIGS. 2A & 2B, FIGS. 3A & 3B show a single, discrete HVFETcomprising an upper transistor section 30 a and a lower transistorsection 30 b on a semiconductor die 21. But in the example of FIGS. 3A &3B, the deep vertical trenches filled with oxide regions 15 b and fieldplates 19 b of transistor sections 30 a and 30 b overlap, or are merged,leaving small, diamond-shaped dummy silicon pillars 33 between thesegmented transistor sections. In this embodiment, a single dummy pillaris centrally located between the four rounded ends of adjacent pairs oftransistor segments over the two sections. In the example shown, forevery N (where N is an integer greater than 1) racetrack segments orstructures in a section 30 of the transistor comprising die 21, thereare a total of N−1 dummy pillars 33.

FIG. 4A illustrates yet another example layout of the vertical HVFETstructure shown in FIG. 1. FIG. 4B is an expanded view of one portion ofthe example layout shown in FIG. 4A. Pillars 17 and oxide region 15 bare just shown for clarity reasons in the expanded view of FIG. 4B. Inthis example, the transistor segments comprising the HVFET ofsemiconductor die 21 are alternately shifted by half of the length ofeach racetrack segment, resulting in racetrack transistor segments thatare alternately associated with upper transistor section 40 a and lowertransistor section 40 b. In other words, each of the transistor segmentsof a row of section 40 a is separated by a pair of the transistorsegments of section 40 b, the pair being arranged in an end-to-endrelationship in the x-direction.

It is appreciated that the alternate shifting of the segments may be anyfraction of the segment length. In other words, shifting of the segmentsis not limited to 50% or half the length. Various embodiments maycomprise segments alternately shifted by any percentage or fractionranging from greater than 0% to less than 100% of the length of thetransistor segments.

In the example of FIGS. 4A & 4B, the dielectric regions 15 b ofalternating ones of the transistor segments in respective sections 40 a& 40 b are merged. In the specific embodiment shown, the rounded ends ofthe transistor segments associated with different adjacent sectionsoverlap or are merged such that field plates 19 b of the adjacentsections are merged at the ends (in the x-direction). Also, the extendedstraight side portions of field plates 19 b of alternating transistorsegments of different sections are merged along a substantial length ofeach segment. It is appreciated that regions 15 b and 19 b may be mergedwith or without a dummy pillar (or isolated dummy silicon pillars)between the respective sections.

FIG. 5 illustrates an example layout of a wafer 50 with die-to-diecheckerboarding of HVFETs 10 a-10 d on semiconductor die 21 a-21 d,respectively. Each of HVFETs 10 comprises a plurality ofracetrack-shaped transistor segments such as that shown in FIG. 1,arranged side-by-side along their width into a substantially squareblock. In this example, HVFETs 10 a-10 d each comprises transistorsegments having a length that extends substantially across the length ofthe respective die 21 a-21 d. In one embodiment, the width of eachsegment is about 13 μm, with the length ranging from about 500 μm to2000 μm. Other embodiments may have lengths greater than 2000 μm. Theblock or stacked arrangement of segments also extends substantiallyacross the width of each die. (Note that the bordered square of each die21 represents the edge of the scribe area between adjacent semiconductordie.) Although FIG. 5 shows two rows and two columns of HVFETs 10 it isappreciated that the die-to-die checkerboarding arrangement shown may berepeated across the entire wafer substrate.

In the example of FIG. 5 adjacent die in a row or a column are orientedsuch that the length of the transistor segments in one die extends inone direction, with the length of the transistor segments in an adjacentdie extending in a second orthogonal direction. For instance, HVFET 10 ais shown with the length of its transistor segments oriented in thex-direction, whereas adjacent HVFETs 10 b & 10 c By orthogonallyalternating the orientation of the transistor segments in eachindividual die 21 across wafer 50 (i.e., checkerboarding) mechanicalstress generated by the long dielectric regions is distributed in twoorthogonal directions, thus reducing warping of wafer 50.

FIG. 6 illustrates another example layout of a wafer with die-to-diecheckerboarding of segmented HVFETs. The example of FIG. 6 utilizes thesame approach as in FIG. 5 of alternating the orientation of thetransistor structures die-to-die; however, in the embodiment of FIG. 6,the HVFET structures are segmented into multiple (e.g., two) sections.For instance, each HVFET that extends substantially across the lengthand width of a semiconductor die 21 is segmented into two sections 30 a& 30 b separated by a dummy pillar 32.

Each of the semiconductor die 21 shown in FIG. 6 has a layout that isthe same as that shown in FIG. 2A for a substantially square die.Similar to the example shown in FIG. 5, adjacent die have transistorsegments that are orthogonally alternating across wafer 50. That is, thetransistor segments in sections 30 a & 30 b of die 21 a and 21 d have alength oriented in the x-direction, whereas the transistor segments insections 30 a & 30 b of die 21 b and 21 c have a length oriented in they-direction.

It is appreciated that the HVFET of each die 21 may be formed withmultiple transistor sections, e.g., greater than 2, each separated byone or more dummy pillars. Furthermore, any of the single die layoutswith multiple transistor sections shown in the examples of FIGS. 2A-4Bmay be utilized in each of the die 21 shown in FIG. 6, with theorientation of the segments alternating die-to-die across wafer 50.

FIG. 7 illustrates an example rectangular layout of a die 25 withcheckerboarded blocks of racetrack-shaped HVFET segments stacked in aside-by-side arrangement of substantially square blocks or sections 36.Adjacent sections in a row or a column are oriented such that the lengthof the transistor segments in one section extends in one direction, withthe length of the transistor segments in the other adjacent sectionextending in a second orthogonal direction. For example, each of therows and columns of die 25 include transistor sections 36 a orientedwith the elongated transistor segments aligned in the x-direction andalternate transistor sections 36 b oriented with the elongatedtransistor segments aligned in the y-direction. The spaces betweensections 36 a and 36 b comprise dummy silicon pillars; that is, thesilicon that forms the dummy pillars is not an active transistor region.

In the embodiment shown, die 25 comprises three rows and four columns oftransistor sections 36. The checkerboarded layout approach shown in theexample of FIG. 7 may be used to produce a single, discrete HVFET on adie of virtually any (within practical limits) rectilinear-shape.

FIG. 8 illustrates an example gate metal routing layout for the dieshown in FIG. 7. The gate metal routing scheme of FIG. 8 is fabricatedusing a single metal layer process with both source and gate metaldisposed on the same planar level. The example shown includes horizontalgate metal bus lines 41 a-41 d that run between each row of thecheckerboarded blocks of racetrack-shaped HVFET segments. For example,gate metal bus lines 41 a & 41 b are shown extending horizontally alongthe top and bottom of the first (upper) row of checkerboarded sections36 of FIG. 7. (It is appreciated that gate metal bus line 41 b may betwice as wide as bus line 41 a due to the face that bus line 41 bprovides a shared conduction path to the polysilicon gate members ofboth the first and second rows of checkerboarded sections.)

Within each row, the sections 36 that have the length of theirtransistor segments aligned in the x-direction have half of thepolysilicon gate members coupled to the top bus line, and a second halfof the polysilicon gate members coupled to the bottom bus line. Forinstance, the upper left-hand block or section 36 in FIG. 8 is shownhaving the polysilicon gate members represented by lines 44 a connectedto gate metal bus line 41 b via contacts 45 a, whereas the polysilicongate members represented by lines 44 b in the same section are connectedto gate metal bus line 41 a via contacts 45 b. Note that each line 44 aor 44 b actually represents the two gate members 18 a & 18 b (seeFIG. 1) of a single racetrack-shaped HVFET segment. Thus, lines 44 arepresent the gate members of the two left-most HVFET segments, andlines 44 b represent the gate members of the two right-most HVFETsegments, in the same section. Note further that each gate member isconnected to a bus line (top or bottom) at one end only.

The gate metal routing pattern shown in FIG. 8 also includes verticalgate metal stub lines 42 that extend approximately half-way across eachrow of checkerboarded blocks. Within each section in which the length ofthe HVFET segments is aligned in the y-direction, half of thepolysilicon gate members are coupled to one stub line, and the otherhalf of the polysilicon gate members are coupled to another stub line.For example, the second section (from the left) in the upper row of FIG.8 shows a bottom half of the gate members (represented by lines 44 c)connected to left-sided gate metal stub line 42 a via contacts 45 c,with a top half of the gate members (represented by lines 44 d)connected to right-sided gate metal stub line 42 b via contacts 45 d.Similarly, the fourth section (right-hand most) in the upper row of FIG.8 shows a bottom half of the gate members connected to gate metal stubline 42 c and a top half of the gate members connected to gate metalstub line 42 d. Note that each gate member of the horizontally-alignedsegments is connected to a stub line (left or right side) at one endonly.

The reason why gate metal stub lines 42 extend only half-way acrossthose sections having their segments aligned in the y-direction (i.e.,horizontally) is to allow the source metal bus lines to extend acrosseach row and contact the source regions of each transistor segment. Thisis illustrated by the example of FIG. 9, which shows a die 25 havingindividual source bus lines 61 that extend continuously across each rowof transistor sections 36 between top and bottom gate metal traces 51.(Metal traces 51 represent the merged metal bus lines 41 and stub lines42 associated with each row.) For example, source bus lines 61 a runscontinuously across the upper row of sections on die 25 to contact eachof the source regions 14 at the top of silicon pillars 17 for each HVFETsegment in the row. In so doing, source bus lines 61 a “snakes” betweenand around stub lines 42, as well as between bus lines 41, all of whichare patterned on the same single layer of metal.

Practitioners in the art will appreciate that by extending stub lines 42approximately half-way across each row, the current handling capabilityof each source bus line 61 is maximized (i.e., minimum notching of lines61). To put it differently, extending stub lines 42 vertically (in thex-direction) a distance other than half-way across each row wouldunnecessarily constrain or pinch current flow across source bus lines 61due to the notching of lines 61 around stub lines 42. Likewise, itshould be understood that by connecting half of the gate members in asection to one gate metal bus (or stub) line, and the other half toanother gate metal bus (or stub) line, electro-migration and resistanceproblems are minimized.

FIG. 10 illustrates an expanded portion of the example layout shown inFIG. 9 that shows one possible scheme for connecting gate metal trace 51with gate members 18 a & 18 b. In this example, via contacts 55 a & 55 bare shown connecting trace 51 with the rounded fingertip portion of gatemembers 18 a & 18 b, respectively. The source region at the top ofpillar 17 located between gate members 18 a & 18 b is shown connected tosource metal bus 61 via contacts 75. (It is appreciated that only twocontacts 75 are shown for clarity reasons.) In an alternativeembodiment, rather than contacting the rounded fingertip portion of gatemembers, gate metal trace 51 may connect along the straight, linearportion of gate members 18 a & 18 b near the rounded fingertip portion.(Note that the field plates are not shown in the example of FIG. 10 forclarity reasons.)

FIG. 11 illustrates an example layout of the rounded end portion of asingle HVFET segment having a structure as shown in FIG. 1. In theembodiment shown, the gate members 18 a & 18 b have been eliminated fromthe rounded end or fingertip portion of the HVFET segment. In otherwords, each of the polysilicon gate members 18 a & 18 b extend laterallyalong the full length of the HVFET segment on opposite sides of the twosubstantially linear portions of pillar 17, but terminate at or near thepoint where pillar begins to curve around at the end portion of thesegment. The rounded end of racetrack shaped pillar 17 is adjoined orflanked on opposite sides by dielectric regions 15, with the gatemembers being completely removed in the area around the end of pillar17. That is, the embodiment of FIG. 11 includes four separate gatemembers: one pair being disposed in the dielectric regions 15 onopposite sides of one linear portion of pillar 17, and a second pairbeing disposed in the dielectric regions 15 on opposite sides of theother linear portion of pillar 17.

In the example of FIG. 11, gate metal bus 51 is shown electricallyconnected with gate members 18 a & 18 b via contacts 55 a & 55 b,respectively, located near the ends of the gate members.

FIG. 12 illustrates another example layout of the rounded end portion ofa single HVFET segment having a structure as shown in FIG. 1. In thisembodiment, the gate members are shown moved back from pillar 17 alongthe rounded end portion of the silicon pillar. For example, polysilicongate members 18 a & 18 b are shown being pulled back distances of d₁ &d₂, respectively, along the opposite sides of pillar 17 at the roundedend portion of the HVFET segment. The gate oxide is therefore muchthicker (e.g., 1 μm) in the rounded or curved portion of the endtermination structure. It is appreciated that the distances d₁ & d₂ aretypically selected to be sufficiently large in order to eliminate gateoxide weakness in the rounded portion of the layout while maintainingadequate distance of gate members 18 a & 18 b from the respective fieldplates (not shown) disposed in dielectric regions 15 a & 15 b.Electrical contact of the rounded fingertip portion of gate members 18 a& 18 b to the gate metal trace/bus may be made as shown in FIG. 10 orFIG. 11.

Although the above embodiments have been described in conjunction with aspecific device types, those of ordinary skill in the arts willappreciate that numerous modifications and alterations are well withinthe scope of the present invention. For instance, although HVFETs havebeen described, the methods, layouts and structures shown are equallyapplicable to other structures and device types, including Schottky,diode, IGBT and bipolar structures. Accordingly, the specification anddrawings are to be regarded in an illustrative rather than a restrictivesense.

We claim:
 1. A high-voltage transistor device comprising: a plurality oftransistor segments, each transistor segment being arrange on a die inan annular layout, each transistor segment including: a pillar ofsemiconductor material having the annular layout with a length elongatedin a first lateral direction and a width in a second lateral directionsubstantially orthogonal to the first lateral direction, the pillarhaving first and second substantially linear portions that extend in thefirst lateral direction and first and second rounded end portions thatconnect the first and second substantially linear portions at respectiveopposite ends, the pillar including a source region disposed at or neara surface of the pillar, a drift region that extends in a verticaldirection through the die, and a body region that separates the sourceregion from the drift region; first and second dielectric regionsdisposed on opposite sides of the pillar, respectively, the firstdielectric region being laterally surrounded by the pillar, and thesecond dielectric region laterally surrounding the pillar; first andsecond gate members respectively disposed in the first and seconddielectric regions at or near a top of the pillar, the first and secondgate member being disposed on opposite sides of the pillar, the firstand second gate members being separated from the first and secondsubstantially linear portions of the pillar by a gate oxide having afirst thickness, the first and second gate members being respectivelyseparated from the pillar by second and third thicknesses of the gateoxide at the first and second rounded portions, the second and thirdthicknesses being substantially larger than the first thickness.
 2. Thehigh-voltage transistor device of claim 1 wherein the first thickness isapproximately 500 Å and the second and third thicknesses are bothapproximately 1 μm.
 3. The transistor of claim 1 further comprisingfirst and second field plates respectively disposed in the first andsecond dielectric regions, the first and second gate members beingcompletely insulated from the first and second field plates.
 4. Thetransistor of claim 1 further comprising a gate metal bus that extendsin the second lateral direction orthogonal to the first lateraldirection, the gate metal bus being electrically connected to the firstand second gate members.
 5. The high-voltage transistor of claim 1wherein the annular layout of the pillar forma a racetrack-shaped ringor oval.
 6. The high-voltage transistor of claim 1 wherein the secondand third thicknesses are substantially equal.